A fuel cell, which generates power by reacting hydrogen with oxygen in air, is required to be supplied with hydrogen as an energy source. The supply of hydrogen to fuel cell equipment can be implemented by several methods as follows:
(a) supplying hydrogen by reforming town gas, LPG, or the like into hydrogen in the vicinity of consumption equipment;
(b) supplying hydrogen directly through a pipeline; and
(c) supplying hydrogen by installing a hydrogen storage container in the vicinity of consumption equipment. In the mean time, various hydrogen supplying methods are studied so as to suit a scale of equipment, intended use, or the like. In particular, fuel cell powered vehicles, in which strict restrictions are posed in terms of space and weight, require a hydrogen storage container that is light-weight and small in volume, as means for supplying hydrogen to fuel cells.
Hydrogen can be supplied to hydrogen consuming equipment such as a fuel cell by using the following types of container:                (1) high-pressure hydrogen storage container;        (2) hydrogen absorbing material container; and        (3) liquid hydrogen storage container.However, the high-pressure hydrogen storage container as mentioned in (1) provides only a small hydrogen storage amount per volume of the container, requiring a larger volume of the container to store a required amount of hydrogen, and thus is not suitable for use in fuel cell powered vehicles or the like.        
As for the hydrogen absorbing material container mentioned in (2) above, a hydrogen absorbing alloy is typically used as a medium. However, since a hydrogen storage amount per weight of the container is small, a problem is posed that the weight of the container becomes remarkably heavy.
Actually, according to the recognition of current situation regarding “the hydrogen energy storage technology such as hydrogen storage devices” of the strategy for development of application technology of solid polymer fuel cells and hydrogen energy as found in the material for the workshop for fuel cell commercialization strategy (Aug. 8, 2001), in order to store 5 kg of hydrogen, a high-pressure hydrogen storage container (70 MPa) is expected to have a weight of 106 kg and a volume 193 L, and a hydrogen absorbing alloy container (3 wt % alloy) is expected to have a weight of 202 kg and a volume of 96 L.
In other words, with the technology currently available, a high-pressure hydrogen storage container is required to have a weight of at least 100 kg and a volume of at least 200 L for satisfying the requirement of storing 5 kg hydrogen, and a hydrogen absorbing alloy container is required to have a weight of at least 200 kg and a volume of at least 100 L. Thus, it is not possible to obtain a container that is capable of storing 5 kg hydrogen and yet has an external volume of less than 200 L and a total weight of less than 200 kg.
Further, regarding the liquid hydrogen storage container mentioned in (3) above, the heat insulation efficiency of the container is required to be high, and there is also a problem that hydrogen tends to be vaporized by heat entering from the outside during the storage and the vaporized hydrogen gas must be discharged out of the container.
Therefore, according to the conventional technology, it is difficult to obtain a light-weight and small-sized hydrogen storage container that is capable of storing hydrogen for a long period of time. In particular, it is necessary for a container for supplying hydrogen fuel to a fuel cell powered vehicle that is considered to be prospective as a low-pollution and energy-saving vehicle to reduce both the weight and volume of the container. Neither the high-pressure hydrogen storage container nor hydrogen absorbing alloy container currently available can be mounted easily on a vehicle for the reasons of space and weight. Also, with the liquid hydrogen storage container currently available, it is difficult to dispose of hydrogen vaporized by heat entering during the halt of a vehicle, and hence it is difficult to employ such a container as a hydrogen container for fuel cell powered vehicles.
A technology that is possibly capable of storing hydrogen for a long period of time with a lightweight and small-sized container is disclosed in Japanese Patent Laid-Open Publication No. Sho 61-252997 titled “Method for Optimizing Storage Capacity Based on Weight of Hydrogen Storage Container Including Hydride Producing Alloy and Hydrogen Container Optimized in Terms of Weight.”
This technology aims at increasing the hydrogen storage efficiency by using high-pressure hydrogen of 100 to 300 bars (10 to 30 MPa) together with hydrogen absorbed by a hydrogen absorbing alloy. However, at a pressure level of less than 30 MPa, it is almost impossible, by using a hydrogen absorbing alloy with αm=100 kg/m3, to obtain a container satisfying the requirements for a hydrogen storage container used for fuel cell powered vehicles, namely a container that has an external volume of less than 200 L and a total weight of less than 200 kg and is still capable of supplying 5 kg hydrogen. Further, presuming that the technology of Japanese Patent Laid-Open Publication No. Sho 61-252997 is applied to a fuel cell powered vehicle, a problem is posed that the hydrogen absorbing material, that is not fixed, will be exposed to vibration during the running of the vehicle and will be unevenly distributed in the container.
In order to obtain a hydrogen storage container satisfying the requirements for the use for fuel cell powered vehicles as mentioned above, it is necessary to provide a hybrid-type hydrogen storage container in which a high-pressure container of 30 MPa or more and a hydrogen absorbing material are employed together. However, when such a high pressure is used, there is posed a problem that, in case of an emergency, such as a collision of the vehicle, active hydrogen absorbing material powder will be dispersed widely to the external system to cause dust explosion, and the risk of inducing a secondary disaster such as a fire will be increased.
Therefore, an object of the present invention is to provide a hybrid-type hydrogen storage container that is easy to mount in a vehicle in terms of the space and weight and that is capable of functioning stably and safely during use and a method of storing hydrogen in such a container. More specifically, an object of the present invention is to provide a hybrid-type hydrogen storage container that makes it possible to charge 5 kg of hydrogen into a container with an external volume of less than 200 L and a total weight of less than 200 kg and that prevents active hydrogen absorbing material powder from being dispersed widely to the external system in case of an emergency.